1. Field of the Invention
The present invention relates to a transmission system that carries out partial response transmission.
2. Description of the Related Art
In recent years, in information processing by a high-end server or a router, the performance in communication between an LSI and an external device is a bottleneck rather than the performance of a CPU inside the LSI. For this reason, the need of a large capacity transmission increases for electrical transmission between back boards or chips that employ a SerDes (Serializer/Deserializer) or the like.
One of methods of permitting the large capacity communication is speed-up of signal transmission. However, in transmission employing as a medium a PCB (Printed Circuit Board) used in a computer or the like, it is not easy to speed up the signal transmission. Increasing the transmission speed results in increasing the frequency of a signal. However, since a frequency band is limited depending on the medium, its waveform largely attenuates in a high-frequency signal, so that it is impossible to detect data correctly by a receiving circuit.
By the way, partial response transmission is known as a technique that allows high-speed transmission while using a limited frequency band, as described in “Partial Response Signaling” by PETER KABAL and SUBBARAYAN PASUPATHY (IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. COM-23, NO. 9 SEPTEMBER 1975). In the partial response transmission, it is possible to narrow the frequency band by accepting intersymbol interference that can be removed through logical processing or the like. In the partial response transmission there are various methods depending on types of intersymbol interference, and methods such as duobinary method, and partial response II method are known.
The intersymbol interference in the duobinary method is expressed as 1+z−1, while the intersymbol interference in the partial response II method is expressed as 1+2z−1+z−2, where z means a delay of 1 bit. Therefore, 1+z−1 in the duobinary method indicates that, data in which data immediately before current data by 1 bit data is added to the current data due to intersymbol interference, is reception data. Therefore, original data can be determined from the reception data, considering intersymbol interference. For this reason, in the partial response transmission, a transfer function of the entire transmission system is adjusted by an equalizing circuit so that desired intersymbol interference is caused.
FIG. 16 is a block diagram showing the configuration of a conventional partial response transmission system. Referring to FIG. 16, the conventional partial response transmission system has a transmission side equalizing circuit 1602, a transmission medium 1603, a reception side equalizing circuit 1604, and a deciding circuit 1606. The transmission side equalizing circuit 1602 equalizes an original data 1601 and then transmits it to the transmission medium 1603. The waveform of a signal transferred through the transmission medium 1603 is largely attenuated and then is received as a weak signal including intersymbol interference by the reception side equalizing circuit 1604. The reception side equalizing circuit 1604 equalizes the signal received from the transmission medium 1603 and then transmits it as a partial response signal 1605 to the deciding circuit 1606. The deciding circuit 1606 decides the original data based on the partial response signal 1605 sent from the reception side equalizing circuit 1604 and then outputs the decided result as a data output 1607.
The transmission side equalizing circuit 1602 has delay circuits 1608 to 1610, multiplying circuits 1611 to 1615, and an adding circuit 1616. The delay circuits 1608 to 1610 are connected in series and sequentially delay the data input 1601 in units of one symbol (1.0 Ts). The multiplying circuits 1611 to 1615 weigh the inputted original data and an output data of each of the delay circuits 1608 to 1610 by multiplying them by predetermined coefficients c0 to cn. The adding circuit 1616 adds output data of the multiplying circuits 1611 to 1615 and then transmits the obtained data to the transmission medium 1603. As a result, the transmission side equalizing circuit 1602 functions as a symbol rate FIR (Finite duration Impulse Response) filter for the data input 1601.
Here, it is assumed that the transfer function of the transmission medium 1603 is C(ω), the transfer function of a combination of the transmission side equalizing circuit 1602 and the reception side equalizing circuit 1604 is E(ω), and the transfer function of the entire partial response transmission system is G(ω). In this case, the following relation (1) is met:C(ω)*E(ω)=G(ω)  (1)
In the partial response transmission system shown in FIG. 16, the characteristic of the transmission side equalize circuit 1602 is specified based on the coefficients c0 to cn, and the transfer function E(ω) is adjusted for the transfer function G(ω) of the entire system to have a desired value.
FIG. 17 is a graph showing an ideal relationship between the transfer function C(ω) of the transmission medium and the transfer function G(ω) of the entire system in the duobinary method. Since this relationship is an example of the duobinary method, the transfer function G(ω) of the entire system is 1+z−1. This transfer function G(ω) in the duobinary transmission has a characteristic of a fan-like form such that the gain becomes zero at a Nyquist frequency fnyq. The transfer function C(ω) of the transmission medium becomes close to zero in a high frequency band due to attenuation caused by skin effect or dielectric loss.
If the maximum gain of the transfer function E(ω) for the combination of the transmission side equalizing circuit 1602 and the reception side equalizing circuit 1604 is normalized to “1”, as shown in FIG. 17, the transfer function G(ω) of the entire system has a curve to make contact with the inner side of the transfer function C(ω) of the transmission medium. In FIG. 17, the gain in the Nyquist transmission is also shown for comparison.
With the configuration as described above, the conventional partial response transmission system transmits data at high speed while accepting intersymbol interference. However, in the system shown in FIG. 16, the output amplitude of the equalizing circuit decreases due to a limitation depending on the frequency characteristic of the equalizing circuit, resulting in great decrease in the level of the partial response signal 1605. The reasons for this problem will be described below.
A frequency characteristic Esym(ω) of the symbol rate FIR filter such as the transmission side equalizing circuit 1602 can be expressed by the following equation (2):Esymb(ω)=Σcne−jωnTs  (2)Now, the maximum value of the gain is normalized by using the following equation (3):Σ|cn|=1  (3)As can be seen from FIG. 17, in the partial response transmission, the frequency at which the gain of Esym(ω) becomes maximum, that is, the frequency at which the transfer function C(ω) of the transmission medium and the transfer function G(ω) of the entire system approach closest to each other, is lower than the Nyquist frequency. Now, paying attention to the gain at ⅔ frequency of the Nyquist frequency, for example, the gain always becomes smaller than “1”, as shown in the following equation (4):
                                                                  E              symb                        ⁡                          (              ω              )                                                =                                                        ∑                                                c                  n                                ⁢                                  ⅇ                                                            -                                              j                        ⁡                                                  (                                                                                    2                              ⁢                              π                                                                                      3                              ⁢                                                              T                                s                                                                                                              )                                                                                      ⁢                    nT                                                                                            =                                                                                      ∑                                                            c                      n                                        ⁢                                          ⅇ                                                                        -                          j                                                ⁢                                                  2                          3                                                ⁢                        n                        ⁢                                                                                                  ⁢                        π                                                                                                                        <                              ∑                                                                        c                    n                                                                                          =            1                                              (        4        )            From this, it could be understood that the entire available frequency band of the transmission medium is not yet used. FIG. 18 is a graph showing an actual relationship between the transfer function C(ω) of the transmission medium and the transfer function G(ω) of the entire system in the conventional system adopting the duobinary method. In the conventional system, the signal amplitude is decreased by the equalizing circuit, and thus the graph of the actual relationship with the transfer function G(ω) of the entire system is different from that of the ideal relationship of FIG. 17, as shown in FIG. 18. As a result, the deciding circuit 1606 can no longer judge a slight potential difference, thus resulting in a failure to accurately transmit data in some cases.
In conjunction with the above description, a signal generating unit is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 8-110370). In this conventional example, an output signal is transmitted from a signal generator in synchronization with a clock signal outputted from a clock signal generator. A digital delay circuit delays the transmission signal from the signal generator for a period equivalent to a predetermined number times of a period of the clock signal. A first amplifier 4 amplifies the delayed signal. A second amplifier sets a rate of a level of the amplified signal and a level of the transmission signal to a predetermined value. A differential amplifier determines a difference between the level of the delayed signal and the level of the transmission signal to a predetermined value. An output signal of the differential amplifier is outputted through a low-pass filter whose cut-off frequency is set to a frequency corresponding to a frequency of the clock signal.
Also, an adaptive equalizer is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 9-321671). In the adaptive equalizer of this conventional example, in order to reduce a circuit scale while maintaining a high data transmission efficiency, adaptive signal processing is carried out to an input digital signal passed through a transmission path to minimize an equalization error. A variable coefficient filter carries out a filtering process on the input digital signal based on preset coefficients. An error detection system detects an equalization error. A coefficient control unit controls the coefficients based on the equalization error. The coefficient control unit includes a deciding circuit to decide whether or not an absolute value of each sample value of the input digital signal is larger than a predetermined value. A coefficient generating section generates the coefficients based on values obtained by giving a polarity according to the polarity of the sample value to the equalization error, when it is decided that the absolute value is larger than with the predetermined value.
Also, a communication system is disclosed in Japanese Laid Open Patent Application (JP-P2003-204291A). In this conventional example, a transmission signal is generated in a semiconductor integrated circuit and supplied to a transmission circuit (equalization circuit) in the semiconductor integrated circuit. A buffering signal obtained by buffering the transmission signal by a buffer and a 1-bit delayed signal obtained by delaying the transmission signal by one bit and inverting the delayed signal are added in a predetermined rate and the addition resultant signal is outputted onto a transmission path. The addition resultant signal transmitted on the transmission path is equalized by an equalization circuit in another semiconductor integrated circuit, and then supplied to a signal decision circuit, which converts it to a digital signal. Thus, by providing the equalization circuit in both of the transmission side and the reception side, the frequency dependence of attenuation of the signal received by the other semiconductor integrated circuit can be made small and an amplification factor of a high frequency component can be reduced in the equalization circuit of the reception side.